Cold seal release films are well known in the art as exemplified in Wilkie et al. U.S. Pat. No. 5,482,780; Wilkie U.S. Pat. No. 5,792,549; Wilkie U.S. Pat. No. 5,981,047; Wilkie U.S. Pat. No. 6,022,612 and Wilkie U.S. Pat. No. 6,074,731. The subject matter of all of the above-identified patents collectively referred to as the “Wilkie patents”) is fully incorporated herein by reference.
Cold seal release films are most commonly used as the outside web or film of a multi-web packaging structure. In common practice, the release film is a clear web that is reverse printed and adhesive laminated to an inner web such as an opaque film (in a candy bar wrap) or a metallized film (in a breakfast bar wrap). After the two films are laminated a layer of cold seal adhesive is applied to the inner web as the web is wound into a roll. The cold seal release film serves the following functions: 1) it provides a transparent web which can be reverse printed so that the decoration is “buried” beneath the clear web to prevent the ink from being worn away; 2) it provides the desired slip properties so that the laminate will run effectively on packaging machines and 3) it provides a surface with low adhesion to the cold seal adhesive so that the laminate roll can be unwound during a packaging operation, with the adhesive remaining on the desired surface of the lamination.
The term “cold seal” refers to a package seal provided at ambient temperature, typically 15-26° C., as opposed to a seal provided with a high-temperature sealant polymer that requires heat and pressure to create the seal. Because of their ability to form seals without heat, cold seal adhesives are ideally suited to the packaging of heat-sensitive products, such as bakery and confectionary products. In addition, employing cold seal adhesives allows faster packaging speeds to be achieved relative to the use of high-temperature sealant polymers.
Cold seal release films typically are multilayer polymeric films having a central core and skin layers located on opposed sides of the core. One of the skin layers (the outer skin layer) is a cold seal release layer designed to permit a cold seal adhesive to be separated therefrom for subsequent use in providing a sealing function. As noted earlier, cold seal release films are typically laminated to another, inside web, and in these latter constructions the opposed skin layer of the release film (i.e., the layer opposite the release layer) is generally a surface-treated (e.g., oxidatively treated, such as by corona or flame treatment), often reverse printed, and then adhesive laminated to the inside web. The inner surface of the inside web is then oxidatively treated to provide a cold seal receptive layer for receiving and retaining a cold seal adhesive composition thereon.
Cold seal release films within the scope of this invention can be utilized by themselves, i.e., without being laminated to another, inside web. In these latter applications the skin layer of the film opposed to the cold seal release layer is commonly oxidatively treated by corona or flame treatment to provide a cold seal receptive layer for receiving and retaining a cold seal adhesive composition thereon.
In all embodiments of this invention, it is necessary to achieve improved cold seal release properties in the cold seal release skin layer to reliably permit the separation of the cold seal adhesive from the cold seal release layer when the film, either by itself or laminated to another inside film, is being dispensed for use in packaging applications.
In selected cold seal release films printed indicia is provided directly on the cold seal release surface. In order to provide good ink receptivity in these latter films it generally has been necessary to oxidatively treat the cold seal release skin layer by corona treatment or flame treatment. Unfortunately, in many prior art cold seal release films the oxidative treatment of the surface of the release layer diminishes the release properties of that layer, thereby resulting in the formation of a product wherein the cold seal adhesive does not effectively and reliably separate from the cold seal release layer without damaging the film.
In a number of the Wilkie patents the specifically disclosed polymeric compositions employed in the release layer in combination with a cross-linked silicone, non-migratory slip agent sold under the trade name Tospearl by Toshiba Company are described as providing good release properties, either with or without corona treatment. Specifically, Wilkie invented polymer formulations for the cold seal release layer. The reason disclosed for adding the slip agent to the release layer in the Wilkie formulations was to reduce the coefficient of friction (C.O.F.) to an acceptable level. In addition to disclosing the use of a cross-linked silicone as the non-migratory slip agent to reduce C.O.F., a number of the Wilkie patents disclose the use of relative minor amounts of other migratory slip/anti-block and anti-static agents to provide this function. There is no disclosure or suggestion that the inclusion of a slip agent or antiblock agent has any possible effect on the release properties of the release layer.
A general concern in the prior art teachings is that oxidatively treating the release surface, although possibly necessary to provide desired adhesion of printing inks to that surface, can deteriorate the cold seal release properties of the film. For example, note column 9, lines 61-64 in Wilkie U.S. Pat. No. 5,981,047.
Wilkie U.S. Pat. No. 5,792,549, identified earlier herein, discloses specific polymer compositions for the release layer, which employ a non-migratory slip agent that is disclosed as being present in an amount sufficient to decrease the coefficient of friction of the release layer. The '549 patent discloses the optional use of non-migratory slip agents in connection with certain formulations, and also other additives, which are described as optional, such as saturated and unsaturated amides having from 16-22 carbon atoms; zinc stearate; calcium stearate; silicone oil; glycerol monostearate; amines; and finely divided inorganic particles. These optional additives are described as being usable in the range of 0-10,000 parts per million. Interestingly there are no specified benefits that are stated to be achieved by using any of these optional additives; nor is there any suggestion that these additives should be employed with or without oxidative treatment of the skin layer.
The thrust of the disclosures in the Wilkie patents is that desirable release properties can be maintained in the release skin layer even if it is necessary to corona or flame treat that layer. There is absolutely no suggestion in the Wilkie patents that the use of any additives can be employed in a manner to actually permit enhancing the release properties of the skin layer by oxidative treatment of the skin layer.